1. Field of the Invention
The present invention relates to an optical modulator used in an optical transmitter of the external modulation type, an ultra high speed optical fiber communication system using the above optical modulator, and an integrated circuit made up of optical elements such as a laser diode and the above optical modulator.
2. Description of the Related Art
Optical modulators of various types have previously been proposed. A typical one of these optical modulators is an optical modulator of the electroabsorption type (refer to an article entitled "InGaAlAs/InAlAs Multiple Quantum Well Optical Modulators" by K. Wakita et. al., IOOC89, 19C2-2). FIG. 1A shows a basic structure of this optical modulator 100. Referring to FIG. 1A, an optical modulator 100 includes an optical waveguide which is formed on a substrate 7, and which has a rib structure of a width of several microns. The optical waveguide has a structure that a light absorption layer 3 made of an undoped layer, i.e., an intrinsic layer is sandwiched between a pair of cladding layers 5-1 and 5-2 which are smaller in refractive index than the light absorption layer 3, as shown in FIG. 1B. The side walls of the optical waveguide are coated with an insulating film. Further, in order to apply an electric field to the undoped layer 3, the upper cladding layer 5-1 and the lower cladding layer 5-2 are doped with a p-type impurity and an n-type impurity, respectively- Thus, a diode of the p-i-n type is formed in the optical waveguide. When a reverse bias voltage is applied across the diode by turning on a switch 12, a strong electric field is applied to the undoped light absorption layer 3.
The light absorption layer is usually made of a bulk semiconductor such as InGaAsP, or formed of InGaAs/InAlAs of a multiple quantum well structure. As shown in FIG. 1B, when an electric field is applied to the undoped layer 3, a spectrum indicating the wavelength dependence of the absorption coefficient of the light absorption layer 3 is shifted to the long wavelength side, on the basis of the Franz-Keldysh effect for the bulk semiconductor or the quantum confined Stark effect for the semiconductor of the multiple quantum well structure. FIG. 1C shows the above shift of the spectrum in correspondence with the structure shown in FIG. 1A. When a wavelength of incident light is set to be .lambda..sub.o, and an electric field is applied to the light absorption layer 3, an absorption coefficient of the layer 3 is increased from .alpha..sub.on to .alpha..sub.off. That is, an ON-OFF control can be made for light propagation, and thereby the intensity modulation of the incident light can be efficiently carried out.
As can be seen from the above, an optical modulator of the electroabsorption type is small in size, i.e., has a length of hundreds of microns, operates on a low voltage such as several volts, and can form an integrated circuit together with another optical element. That is, this optical modulator is very advantageous.
However, in the optical modulator of the electroabsorption type, there arises a problem that change in refractive index of the light absorption layer due to the application of an electric field thereto brings about phase modulation. Thus, the spectral width of output light from the modulator is enlarged, and the so-called chirping phenomenon occurs.